The invention relates to a method of manufacturing ceramic powders having the perovskite structure by means of a reaction in an aqueous solution.
Ceramic powders having the perovskite structure are an important starting material for the manufacture of widely varying electronic components such as, for example, multilayer capacitors or non-linear resistors having a positive temperature characteristic. The demand for continuously improving electronic components as well as a better process control in the manufacture of these components imposes high requirements on the starting materials.
The mixed-oxide method is still predominantly used in the industrial production of perovskite compounds such as, for example, BaTiO.sup.3 powders, to produce such powders barium carbonate and titanium dioxide are mixed in mills and converted into BaTiO.sup.3 in a solid-state reaction at a temperature .gtoreq.1000.degree. C. A disadvantage of this method is that high reaction temperatures and qrinding processes are required. Aggregation of the powder particles is brought about by calcining at temperatures of approximately 1000.degree. C. and higher, as a result of which coarse-grained powders are obtained These powders must be reduced in size by means of grinding processes. However, it is very difficult to obtain particle sizes of &lt;1.mu.m. Due to aggregation of the particles an undesirably wide particle-size distribution is obtained. Moreover, due to abrasion in the grinding processes impurities are introduced into the powder.
DE-OS 35 26 674 describes a method of manufacturing a mass containing perovskite compounds, in which method perovskite compounds of the general formula ABO.sub.3, for example, BaTiO.sub.3, are used, according to this patent the following hydrothermal reactions are employed.
A. Reaction of a mixture consisting of an alkoxide containing an A-group element with an alkoxide containing a B-group element and with water at a temperature of approximately 80.degree. C., and the subsequent conversion at reaction temperatures of 150.degree. C.
B. Reaction of a hydroxide containing the A-group element with an alkoxide containing the B-group element, as described under A.
C. Reaction of a hydroxide containing the A-group element with a hydroxide, containing the B-group element, as described under A.
D. Reaction of a hydroxide containing the A-group element with a salt containing the B-group element, as described under A.
E. A mixture containing the A-group element and the B-group element as a salt, is converted with alkali hydroxides, applying the conditions as described under A.
In the reactions under A and B alkoxides are used which are very sensitive to hydrolysis; this has the disadvantaqe that storage and preparation must take place under an inert gas atmosphere exluding H.sub.2 O, O.sub.2 and CO.sub.2. Thus, this process is relatively costly. The reactions described under B and C have the additional disadvantaqe that the starting materials are not made to react in a homogeneous solution, which leads to the formation of inhomogeneities in the end product.
The reaction as described under E has the disadvantage that alkaline compounds are required for the reaction. It has been found that the electric properties of ceramic powders manufactured in the above-described manner do not meet all requirements.